Turn to the Experts:

COMMERCIAL HVACAIR-HANDLINGEQUIPMENT

Fans:FeaturesandAnalysis

Technical Development Program

Technical Development Programs (TDP) are modules of technical training on HVAC theory,system design, equipment selection and application topics. They are targeted at engineers anddesigners who wish to develop their knowledge in this field to effectively design, specify, sell orapply HV AC equipment in commercial applications.Although TDP topics have been developed as stand-alone modules, there are logical groupings of topics. The modules within each group begin at an introductory level and progress toadvanced levels. The breadth of this offering allows for customization into a complete HVACcurriculum - from a complete HV AC design course at an introductory-level or to an advancedlevel design course. Advanced-level modules assume prerequisite knowledge and do not reviewbasic concepts.

Introduction to HVACPsychrometriesLoad Estimating

ControlsApplications

The heart of any air-handling system is the fan. Fans may consume more energy in a typicalHVAC system than the compressors! It is extremely important that the correct type of fan be chosen for the application. This TDP module will describe fan characteristics and performance inorder to provide designers with the knowledge to select and apply the proper fan for variousHVAC situations.

2005 Carrier Corporation. All rights reserved.

The information in this manual is offered as a general guide for the use of industry and consulting engineers in designing systems.Judgment is required for application of this information to specific installations and design applications. Carrier is not responsible forany uses made of this information and assumes no responsibility for the performance or desirability of any resulting system design.The information in this publication is subject to change without notice. No part of this publication may be reproduced or transmitted inany form or by any means, electronic or mechanical, for any pu rpose , without the express written permission of Carrier Corporation .

FANS: FEATURES AND ANALYSIS

IntroductionIn the HVAC industry, the fan is one of the most important components in the heating andcooling system. It is also one of the easiest components to misapply because of all the types andarrangements available. Fans are important because they can consume more energy than the airconditioning compressors in a building.The fan itself consists of a rotating impeller and a fan scroll housing to collect and direct theairflow in the direction desired. A fan operates on the same basic principle as a centrifugal pump,converting rotational mechanical energy into fluid or air energy. The energy created by the fan isdetermined by the total pressure increase (velocity pressure + static pressure) of the air passingthrough the fan.The fan industry is based on technology that is, for the most part, not new. The basic fluidmechanics governing fan aerodynamic design and performance have been well known for decades. Standards for the construction, testing, and performance rating of fans are well establishedand strictly adhered to by most fan manufacturers. Because of this, fans are often treated likecommodities instead of important pieces of HV AC equipment that should be carefully andthoughtfully selected.Centrifugal fans are the most widely used type of fan in the HVAC industry. For that reason,this TDP module is geared primarily towards centrifugal fans .We will examine fan construction, types of fans , the fan laws that govern centrifugal fan performance, stability factors , and the effects of field application of fans (system effect).At the end of this TDP, the reader should have an understanding of the technical issues involved to properly select and apply the correct fan for a commercial HVAC system.

Commercial HVAC Equipment

FANS: FEATURES AND ANALYSIS

Fan TypesA fan is a device used to produce a flow of air. Fans are classified into two general types,centrifugal and axial.

Centrifugal FansCentrifugal fans are classified according to impeller (wheel) blade design. The most commonly used impeller designs for centrifugal fans for comfort air conditioning are forward- curved,backward-inclined, and airfoil. Impellers and their applications will beAir is discharged at a right angle to fan shaftcovered in this TDP module.The air is drawn in through one orboth sides of the centrifugal fan impeller and is discharged at a rightangle to the fan shaft. A centrifugalfan impeller is usually enclosed in ahousing also called a scroll. The air isdischarged from the impeller throughthe outlet in the fan housing. Whenthis housing is mounted inside an insulated cabinet, it comprises the fansection of an air handler. Refer toTDP-611 , Central Station Air Handlers for further information.

Figure 1Centrifugal Fan Configuration

Plenum FansWhen centrifugal airfoil impelleris applied without the housing, and islocated inside a cabinet, it is called aplenum fan . Plenum fans will be covered also in this TDP module.

Single-width, single-inlet airfoil impeller design ,

FANS: FEATURES AND ANALYSIS

Axial Fans (In-line)

In an axial fan, air flows and is discharged parallel to the fan shaft, not at right angles to thefan shaft as with a centrifugal. Axial fans are classified as propeller, tube axial, and vane axial.These fans (with the exception of the propeller) have a tubular configuration, hence the term "inline." Vane, or tube axial, fans can be driven with an internal direct connected motor or an external shell mounted motor.There are several variations on anaxial or in-line fan that we will coverin this TDP module. The first utilizesa centrifugal impeller in an in-linecylindrical tube configuration. Air isdischarged from the impeller andturns 90 degrees in the shell beforeflowing through straightening vanes.

Air is discharged parallel to the fan shaft

The second is a hybrid between a

centrifugal and an axial. It is called amixed flow fan. Air is discharged offa centrifugal type impeller that hasangled blades. The air then exits thecylindrical tube that houses the fan.Figure 3Axial Fan ConfigurationPhoto courtesy of Bany Blower

Centrifugal FansShown here are the components of a double-width double-inlet (DWDI) fan assembly. This isessentially two single-width fans , side by side, with two inlets and a single outlet or dischargewith no partition in the scrollhousing. A single-width sinDouble-Widthgle-inlet fan (SWSI) wouldhave a single inlet and take upless space from a width standpoint, but would need to be of...., . ' .Igreater diameter than theI..._ '.,DWDI to move the same volI/ume of airflow. SWSI fans are',-.... I ,often applied where it is necHousing '-Outlet Areaessary to mount the fan motorSide Sheetfor DuctConnectionout of the air stream, for example corrosive air. DWDIdesigns are more common mHV AC equipment.

..... ,

Figure 4Centriji1gal Fan Construction and Terminology (DWDI Fan)

Commercial HVAC Equipment

FANS: FEATURES AND ANALYSIS

The following are some of the basic components that make up the fan assembly: Bearing support - supports the fan shaft bearing on both inlet sides of the scroll housing Inlet collar - attaches the bearing support to the fan housing Inlet cone- an aerodynamic inlet design used to reduce entrance losses of the fan (used onbackward inclined and airfoil fans) Impeller - the round assembly containing multiple fan blades that is attached (keyed) to thefan shaft. The impeller (also called the wheel) spins to move the air from inlet to discharge. Fan blades - parts of the impeller that are mounted to the hub that force the air to move;the types of blades define the fan capability and application Wheel backplate or hub- supports the impeller blades and allows the fan wheel assemblyto mount to the fan shaft (not shown) Fan shaft - the round piece of precision-ground steel that the wheel is fastened to that intum is driven by the fan motor normally through pulleys to spin the wheel. The fan shaftmay also be direct coupled to the motor. This is called direct drive. Fan scroll housing - the fan scroll housing is the sheet metal wrapper that leads the airstream from the fan wheel inlet to the discharge outlet Cut off - a plate that is positioned under the blast area that is designed to give the fan thedesired discharge characteristics and performance Blast area- the open discharge area of the fan scroll housing, which is above the cutoff Fan outlet - the part of the fan scroll housing that will connect to the discharge ductwork

Impeller DesignShown here is a radial fan impeller (wheel) with straight blades. We have chosen to showthis straight impeller design as a way to examine the vectors related to the fan blades. Thisstraight blade designed centrifugal impeller is often used for material handling in industrialplants. Later we will show the curvedVR - - -Resulting velocity in the scrollimpeller designs and vector character/ Radial Velocityistics for the centrifugal fan typesV1Bladeused in HV AC applications. In this~ V2and other blade vector diagrams weTangential Velocitywill show later, Vl represents the ra(Tip Speed)dial velocity component leaving thewheel, V2 represents the tangentialvelocity leaving the wheel. V2 isequal to the tip speed of the blade. VRis the velocity resulting from the Vland V2 vectors and is the velocityrelative to the fan scroll housing. Therelative length of VR is a function ofthe blade design and the tip speed Figure 5Impeller Velocity Vectors

FANS: FEATURES AND ANALYSIS

Tip speed is a function of fan RPM. Certain impeller designs can be spun at lower speed thanothers to produce the same airflow. As an example, as we will see, a forward-curved impellerproduces a large VR relative to tip speed, versus an airfoil. So a fan with a forward-curved impeller can be operated at a lower rpm than the airfoil.For all fans , the impeller type used develops a total pressure difference over the inlet and outlet airstreams. The total pressure (PT) rise comprises twomain components. The first is static pressure (Ps),which depends on the blade profile, number of blades,pitch (angle), and other aerodynamic characteristicsof the fan impeller. The second component is velocity(dynamic) pressure (Pv), which develops due to velocity or kinetic energy imparted to the air stream.Static pressure is the "bursting" pressure in all directions in the ductwork created by the fan. Velocitypressure is the pressure in the direction of airflow.

Velocity PressureFigure 6Static and Velocity Pressures

Forward-CurvedOn a forward-curved centrifugal fan , the impeller blades arecurved as can be seen here. The air leaves the wheel (VR) at a velocity greater than the tip speed (V2) of the blades. Tip speed is afunction of wheel rpm. Since this impeller blade design results insuch a large VR, the wheel rpm can be reduced and still produce acomparable airflow to other blade designs. Airfoil and backwardinclined, which we will discuss, must be rotated at higher speed.At a given airflow capacity, the forward-curved fan impeller canoften utilize a smaller diameter wheel.Because the forward-curved fancan be rotated at slower speeds and isused for lower static pressures, it is alightweight design and is thereforeless expensive. The fan wheel has 24to 64 shallow blades with both theheel and the tip of the blade curvedforward. This fan is used primarily forlow-pressure HV AC applications.Forward-curved fans are best appliedoperating at static pressures up to 5.0m.wg.

Commercial HVAC Equipment

FANS: FEATURES AND ANALYSIS

Forward-curved wheel designs,

like all centrifugal fan wheels, shouldbe used in clean environments. Operating in dusty or dirty environmentscould result in an unbalanced fanwheel.

Overloading type fan

-

Horsepower will continue to

rise with increased cfm andcan overload the motor

Fan Horsepower

Forward-curved centrifugal fans

have an overloading horsepowercharacteristic as the airflow throughthe fan increases at a constant rpm.This is why forward-curved centrifugal fans are called overloading typefans .

TypicalForward-Curved rpmLine

cfm ...

A typical example of an overload- Figure 8

ing situation is where a forward- Forward-Curved Centrifugal Fan Characteristicscurved centrifugal fan is used fortemporary heat duty in an unfinished building. If the ductwork is not completed, the resistance ofthe duct system may be lower than design, and the fan can deliver more air than required and mayeventually overload the motor.It may be noted that the static pressure-cfm curve of a fan using a forward-curved wheel has asomewhat gradual slope and also contains a "dip." That is how you can recognize a forwardcurved application, versus an airfoil or backward-inclined impeller application, which will have asteeper slope and no dip. The dip in the curve of the forward-curved centrifugal fan is to the leftof peak pressure. When making fan selection with a forward-curved centrifugal fan, it should bemade to the right of the dip to avoid unstable fan operation.Centrifugal Forward-Curved HousingThe housing is an aerodynamic scroll configuration, which promotes the conversion of velocity pressure from the impeller to static pressure for the duct system. The fan housing width willvary based on whether or not the fan wheel inside is a single width single inlet, or double widthdouble inlet type. With forward-curved fans , the scroll design is critical for the conversion of velocity pressure to static pressure and the inlet design is of secondary importance.

Airfoil and Backward-Inclined

The airfoil impeller is shown below. The airfoil blades have a cross section similar to an airplane wing. Airfoil blades have a thickness that forward-curved and backward-inclined blades donot. A backward-inclined impeller is a thinner (single thickness) bladed airfoil and has an efficiency only slightly less than an airfoil. A backward inclined (BI) impeller will have singlethickness blades that are inclined away from the direction of rotation. Fans with airfoil and backward-inclined impellers have the highest efficiency of all centrifugal fans .Each airfoil and backward inclined impeller uses approximately 8 to 18 blades inclinedbackward from the direction of rotation. Because of this, the air leaves the wheel (VR) at a velocity less than the blade tip speed (V2). For a given duty, fans with these impellers will have thehighest wheel speed. Fans with airfoil impellers are designed to operate, depending on the fan

FANS: FEATURES AND ANALYSIS

Blades are curved away from direction of rotation

Static pressure up to 10 in. wg8 to 18 blades

High rpm (1500 to 3000 rpm)

oo

Figure 9Airfoil Wheel Design

Backward-inclined and airfoil fan

wheelsareconsidered"nonoverloading" because they have thecharacteristic of almost constantpower consumption for the same operating speed (rpm). Some engineerslike to use airfoil instead of forwardcurved centrifugal fans (when thechoice exists) for that reason, eventhough they cost more than forwardcurved fans. In those areas of applications where either type of fan couldbe used, it is prudent to make bothselections and compare.

size and the manufacturer, at static

pressures up to 10 in. wg or higher.Fans with airfoil impellers are nottypically used at the static pressureswhere forward-curved centrifugalfans are the best choice such as lessthan approximately 5 in wg.Typically, fans with airfoil impellers are used primarily in large airhandlers for systems having relativelyhigh static pressure requirements.Since they are capable of higher staticpressures and operate at higherspeeds, they are more ruggedly built,which adds to their cost and weight. Non-overloading-

Horsepower will peak

and begin to drop off

~ ~~~Rf~~~~~~~~

~ 8ifg;~;~~~~++\~ H4+H~FH+~~~4+H+~~K+I

~ H++r~~H+~~~.+rh++M~~

Fan Horsepower

()

:;:;

ro

I+J'rH~H+rH~~~~+H+rH+I

U5 ~L~~

Typical Airfoilrpm Line

cfm ...Figure 10Ailfoil Centrifugal Fan Characteristics

Centrifugal airfoil and backward-inclined housing

The housing design for an airfoil and backward inclined centrifugal fan is similar to the housing for a forward-curved. However it is more critical to maintain close clearance and alignmentbetween the impeller and the inlet in order to maintain the high efficiency.

Commercial HVAC Equipment

FANS: FEATURES AND ANALYSIS

Plenum FanPlenum fans use non-overloading, single-width single inlet (SWSI) centrifugal airfoil impeller designs constructed of heavy gauge steel with each blade continuously welded to the wheelcone. The fan and its motor operate un-housed within a pressurized plenum or cabinet. When thistype of fan utilizes a motor external to the plenum, it is called a plug fan. In a central station airhandler, the plenum is the unit casingprovided by the manufacturer. Ductwork is connected directly to theplenum without an intermediate transition. In essence, plenum fans usetheir plenum enclosure as a fan scroll.Plenum fans do not discharge airdirectly off their impeller and into adischarge duct. The fan pressurizesthe plenum it is located in and air isdischarged out of the various openings, which are typically field cut intothe plenum. For this reason, fan discharge noise is absorbed in theplenum cabinet. This makes the plenum fan ideal for acousticallysensitive fan applications.

Characteristics: Single-Width, Single-Inlet (SWSI) Operate at static pressures up to 10 in. wg Best application with limited space or when multipleduct discharge is desired

Figure 11Plenum Fan CharacteristicsCourtesy of Barry Blower

Notice the developed

inlet cone design to the single inlet airfoil wheel. Thisallows the fan to efficientlydevelopstaticpressurewithin the wheel.

Inlet Cone

An important reason that

makes plenum fans so popular is that they allow forflexibility in discharge arrangements. The plenum fanmay also reduce the spacerequired in the mechanicalroom for the air-handlingunit and the discharge ductwork.

Figure 12Plenum Fans with Cabinets

Commercial HVAC Equipment

FANS: FEATURES AND ANALYSIS

Axial (In -line) Fans

Axial (also called in-line) fans are often used for high cfm, low to medium-static applications.The design of the in-line fan allows for direct connection to supply or return ductwork, which cansave space in the mechanicalroom. Axial fans are often ap- Use for high cfm applicationsplied as return fans as part of a In-line space savers with no cabinetsupply-return fan system. Theyare also used for exhaust air Often used in industrial AC and ventilation applicationsapplications and can even be Impeller similar to prop fans but blades are more aerodynamicfitted into factory fabricated Often used for return fans in AC applicationsair-handling units for supplyduty.PropellerType

One major difference from

centrifugal fans is that air isdischarged parallel to the shafton an axial fan.

Impeller

Propeller fans are a type of

axial fan that is not typicallyducted. They are used for mov- Figure 13ing high volumes of air at very Axial (In-line) Fanslow static pressures. Propeller Photo courtesy ofBany Blowerfans operate at low rpm and arean inexpensive design.Tube axial fans use a fan design with a propeller type impeller (but with a more aerodynamicconfiguration) inside a cylindrical tube. They may come with a sound attenuating accessory tohelp reduce noise levels. Tube axial fans offer a greater efficiency than propeller fans and can beducted.Vane axial fan designs are similar to tube axial but incorporate guide (straightening) vanes onthe discharge to help redirect the air and improve efficiency. Some vane axial fans have a moveable impeller blade capability. Thepitch or angle of the blades can bevaried based upon the static pressure and airflow required. The bladeangle can be changed manually orautomatically.The impeller design of an axialfan wheel is similar to a propellerexcept that the blades are moreaerodynamic.

Commercial HVAC Equipment

FANS: FEATURES AND ANALYSIS

For instance, another version of an

in-line fan actually uses a centrifugalimpeller. It is called a tubular centrifugal. Even though this fan uses acentrifugal impeller, its overall tubularconfiguration resembles that of an axialso we have placed it in the axial sectionofthis TDP module.

Efficient because of centrifugal wheels

Air is discharged from the wheel , then isredirected through straightening vanes asshown here

A centrifugal impeller is mounted

Straightening Vanesin an in-line (tubular) housing and air isredirected out via straightening vanesjust as in a vane axial. A tubular cen- Figure 15trifugal takes advantage of the Tubular Centrifugal In-Line Fanefficiencies of a centrifugal impeller Photo Courtesy ofBan y Blowerand the space-saving configuration ofan in-line design.Mixed Flow FanMixed flow fans can be used forreturn air, supply air, or general ventilation applications where low soundlevel and good efficiencies are important.

Axial

The mixed flow wheel design

Figure 16combines the working properties ofboth axial fans and tubular centrifugal In-line Fan Typesfans . Mixed flow fans draw the air in Photo Courtesy of Greenheck.and exhaust it in a more linear fashion, resulting in a more efficient system, which, in tum, reduces motorhorsepower requirements.Another advantage to the mixedflow design is the reduced sound.Mixed flow fans run at lower rpm todeliver the same amount of airflow.As a result of slower wheel speed,sound generation is reduced significantly.

FANS: FEATURES AND ANALYSIS

Axial Fan Housing Design

The housing design for axial fans is a cylindrical tube. The tube axial and vane axial fans arebuilt with close tolerances from the blade tips to the sidewall of the tube or shell. On the tubularcentrifugal and mixed flow design, theclearance to the wheel is not as critical since the air comes off theseimpellers and must be turned to exitMotorImpellerthe shell.In an axial fan, the motor may beinternal to the shell in a direct driveconfiguration or externally mountedon the shell in a belt drive configuration. The benefits to direct drive arethere are fewer components to wearsince there are no pulleys and belts.Also, the overall unit can be morecompact than the equivalent beltdriven model. With direct drive, themotor is in the air stream, which helpsthe efficiency by cooling the motor.Belt drive units position the motorout of the air stream for easy accessand service. Also, system airflow adjustments can be accomplished bysimply changing pulleys. Dischargesound levels are also less with a beltdriven model

Figure 18Direct Drive Axial FanPhoto Courtesy ofGreenheck

ImpellerBelt Drive

Vane axial, tube axial, and mixed

flow fans are typically controlled byVFDs when used in variable air volume systems. Other means of controlinclude the use of inlet vane dampersand/or control dampers. The use of Figure 19both of these devices is on the declineBelt Drive Axial Fanin favor ofVFDs.Photo Courtesy ofGreenheck

Commercial HVAC Equipment

FANS: FEATURES AND ANALYSIS

AMCA Fan Classes

AMCA (Air Movement and Control Association)isaninternational,non-profitorganization, dedicated to the certification ofperformance ratings on fans, louvers, dampers,and other air-handling equipment. AMCA provides fan manufacturers an independent thirdparty verification of their performance ratings.There are eight certified programs coveredby AMCA. For the purposes of this TDP module, the most important programs are AirPerformance and Sound Perf01mance.Figure 20AMCA

AMCA Class

Maximum SystemStatic Pressure

4 in. wg

II

7 in. wg

Ill

12 in . wg

AMCA categorizes centrifugal fans into

threeperformance/constructionclasses(Class I, II, and III) based on certain definedoperating criteria. Each different class corresponds to a certain maximum total pressureat which the fan will operate. This chartshows the maximum pressure limits for eachfan class.

Figure 21AMCA Fan Classes

Fan construction class ratings are

based on the outlet velocity from the fandischarge and the total system staticpressure. Most fan discharge velocitiesare designed around 2500-3000 fpm.To go to a higher class, manufacturers may use different methods. Somemay increase metal gauge, shaft diameter, add tip material, change to a higherstrength material, etc. The bottom line isthat the added loads of the higher speedsmust be accommodated in the design.

C)

~c

16 ---11514 - - - - - - r --~---+----+---+

~131--------------TyplcaiCiaull

12 ~~rlaHio

CU.!!!_

~ 11

(/)~

....

101----~~.-~J---~~~r-~~--~-----r~

O..g u

:;:; 8

~ 7

Cf)

6 +-- --'--..-.-

If you run a Class II wheel in a Class

I condition, it should last longer than aClass I wheel in the Class II conditions.A Class II wheel running in Class IIconditions will not necessarily lastlonger than a Class I wheel in Class Iconditions.

1000

2000

3000

4000

5000

6000

7000

Outlet Velocity (fpm)

Figure 22AMCA Centrifitgal Fan Construction Class

Commercial HVAC Equipment

FANS: FEATURES AND ANALYSIS

The cost of Class III construction is usually prohibitive to be used for Class I conditions.Here are two examples of how to detennine fan class using the chart in Figure 22. If the fandischarge velocity is 3000 fpm and the total system static pressure is 6 in. wg, the operating conditions fall within the AMCA Class II range and a Class II fan should be considered for thisapplication. If the fan discharge velocity is 2500 fpm and the total system static pressure is 3 in.wg, the operating conditions fall within the AMCA Class I range and a Class I fan could be usedfor this application.

Preferred alternatives to multi-rating tables are fan curves. Figure 24 shows an example of afan curve from years ago. The cfm was plotted on the horizontal axis, with static pressure (in.wg)plotted on the vertical axis. This happens to be a high static capable airfoil type fan curve. At theintersection of these two required values, the fan speed may be read from the family of speedcurves.As an example, a selection at26,000 cfm and 6 in. wg static pressure requires a fan rpm of 1800.The bhp was often represented onyet another curve (not shown here).

Cl:: 16!--<-

.Q)

.....

~ 12

rn

There are several advantages to

selecting with curves. Static efficiency lines (SE) may be provided asshown. Maximum static efficiency(MSE) is measured in percentages;MSE defines the most efficient operating range of the fan, but does notdefine an actual value of efficiency.Below curve C, in this example, theactual static efficiency drops to lessacceptable performance limits.

TypicalSpeedCurve(rpm)

StaticEfficiencyLine

c..0

.;:::(1)

( /)

~40

1-

..,

GOO

00

Figure 24Centrifugal Fan Curve Example

Commercial HVAC Equipment

FANS: FEATURES AND ANALYSIS

Our example point at 26,000 cfm and 6 in. wg fell on the 90% SE line, so that is an efficientoperating point.When selecting the type of fan in an air conditioning system, the goal is to keep the energyinput low, while having a stable selection. For centrifugal fans, the forward-curved impeller is thelowest in static efficiency at approximately 65-70%. Thebackward-inclined is a higher efficiency fan at 75 to 80% The best fan selectionsstatic efficiency. The airfoil impeller, which is a refinement ofthe backward-inclined design, is the most efficient at approximately 80-85% static efficiency.

Fan LawsFan laws are a series of equations that can predict the performance of fans at any operatingcondition. However, to use the fan laws, a known condition of operation is required as a startingpoint.Fan laws predict the airflow (cfm), static,velocity or total pressure and required brakehorsepower (bhp) at varying fan speeds (rpm)and air densities. Designers of HV AC systemsare usually interested in knowing the behaviorof a given fan operating within a given ductsystem. Under these circumstances the following fan laws are applicable.

The most commonly used fan laws in

simplified form are:

cfm varies DIRECTLY with rpm

Ps varies with the SQUARE of the rpmbhp varies with the CUBE of the rpmFigure 25The Three Main Fan Laws

Here are the three most widely used fan laws. Others involving density and air temperaturechanges from standard are listed in the Appendix.

First Fan Law:

o cfm varies directly with the fan speed.

[cfm2]

cfmi =rpm I and rpm2 = rpm J *

cjm 2 rpm 2cjm 1

Second Fan Law:

o Total system static pressure or system resistance varies as the square of the fan speed.

~=[ cjm,l2Ps2

cfm2

rpm / ]2 and Ps2

[ rpm2

= Ps , * [-rp_m_2 ]2rpm 1

Third Fan Law

o Brake horsepower varies as the cube of the fan speed.

Commercial HVAC Equipment

FANS: FEATURES AND ANALYSIS

Density EffectsAt a steady fan rotation (rpm), a fan is aconstant displacement device. It will move thesame airflow (cfm) regardless of the density ofthe air being handled. Fans are rated based onstandard air conditions. Standard air has adensity of 0.075 lb/fe . This density is thesame as that of dry air at 69.8 F and a barometric pressure of29.92 in. Hg or 14.696 psia.

Air Density Factors

Altitude(ft.)01000200030004000500060007000

701.000.964.930.896.864.832.801.772

Temperature100.946.912.880.848.818.787.758

For example, if a fan can move 5000 cfm

at standard air conditions, at the same speed, it7<1(1will move 5000 cfm of air at 200 F. However, the density of air at 200 F is 80 percent Figure 26of the standard air density at 69.8 F, therefore Fan Laws - Air Density Factorsonly 80 percent of the horsepower is requiredto move it.

200.803.774.747.720.694.668.643.620

300.697.672.648.624.604.580.558.538

Because the mass flow of air at 200 F is only 80 percent (0.803 from table) of the mass flowat 69.8 F, the fan will create only 80 percent of the velocity and static pressures. The reduction instatic pressure will be proportional to the horsepower; therefore, the static efficiency of the fanwill remain unchanged. For example as shown above, at 6000 feet above sea level, the density ofair at 69.8 F is approximately 80 percent (0.801 from table) of standard air density. At this elevation, the fan would perform in the same as described when handling air at 200 F at sea level.

Other Fan Laws

It is important that the system designer understand

the basic characteristics of fans . Once the required airflow is known (recirculation or exhaust), the best way toevaluate the system is at standard air conditions. Adjustments can be made in the system to ensure the desiredmass flow is being provided when the density of the airhandled by the fan differs significantly from standard airdensity fan ratings.

Example: Using the Fan Laws

An engineer estimates that hisduct static resistance will be 2.5 in.wg for an 8500 cfm air-handling system installed in a nursing home.Filter resistance is 0.50 in. wg, cooling coil resistance is 1.1 in. wg, andheating coil resistance is 0.4 in. wgfor a system static pressure of 4.5 in.wg. An airfoil centrifugal fan is selected and submitted with thefollowing fan curve. The resultingrpm is 2539 and the bhp is 11 .2.

Commercial HVAC Equipment

FANS: FEATURES AND ANALYSIS

During the actual installation of the duct system, the architect decided to change the ceilingfrom a flat suspended ceiling to a more "aesthetically pleasing" tray ceiling. This requires fouradditional elbows and other duct changes to be added to the supply and return ductwork, whichraised the supply duct static resistance another 0.75 in. wg by the engineer' s calculation. The newtotal static pressure will be 5.25 inches. Using the fan laws, what will the new fan rpm and motorhorsepower be?Using the second fan law to solve for the new rpm:P5 1 = [ rpm 1Ps2rpm 2rpm 2 = rpm 1

After calculating the new rpm and horsepower, we need to make sure that we have not exceeded the fan or motor's capability. A quick check of the maximum fan rpm shows us that wehave not (Class II max rpm 2950). The original fan selection required 11 .2 bhp so a 15 hp motorwas selected. The new horsepower requirement is 14.1 hp, so the motor wi ll not need to bechanged.

Commercial HVAC Equipment

FANS: FEATURES AND ANALYSIS

Analyzing this one step further, what would be the additional energy cost because of the architect's change? Given the following:Operating hours per day

18 hours

Days of operation

365 days

Additional motor horsepower

2.9 bhp

Motor efficiency

92%

Electrical rate

$0.10435 per kWh

18 hrs * 365 x 2.9 bhp * 0.746kW I bhp * $0.10435 / kWh

.92Additional energy cost per year = $ 1,6 12

System Curve, Fan Stability, System Effect

System CurveAn air system may consist simply of a fan with ductwork connected to the inlet or the discharge, or both, as in an exhaust system. A more complex system could include a fan, supply andreturn ductwork, cooling and/or heating coils, filters , air mixers, mixingboxes, diffusers, zoning terminals,dampers, sound attenuators, etc. Thefunction of the fan is to provide therequired energy to the airstream toovercome the resistance to flow imposed by all the system components.

1.

The component manufacturer 2. Coil

usually provides the pressure loss or 3. Duct Elbowsflow resistance for individual compo- 4 Supply Duct5. Supply Diffusernents. In addition, the pressure losses 6 _ Return Grillefor the duct system must be deter- 7. Return Ductmined. The procedure for determiningduct resistance is discussed in TDP- Figure 29504, Duct Design Fundamentals. System Resistance ComponentsLater in this module, the effects offield connections will be discussed to assist the designer in evaluating the effect of these items onfinal fan performance. The summation of all these resistances establishes the required fan totalstatic pressure.The system curve defines the volume flow rate versus pressure characteristics of the duct system in which a fan will be installed. For most app lications, the volume flow rate to pressurerelationship of a system is governed by the following equation, often called the "duct law. " Notice it is the second fan law.

ua

Commercial HVAC Equipment

FANS: FEATURES AND ANALYSIS

~ __ [

cfm 1 ]

Ps 2

cfm 2

= rpm

[rpm 1 ]

cfm 2 ]

or Ps 2 -_ p s t * [ - cjm 1

Once the system designer has determined the total system static pressure loss (Ps) for one airflow (cfm), it is very easy to calculate the corresponding pressure loss for any other flow rate.The system curve is not included on the fan performance curve when it is issued from the fanmanufacturer since its determination must be left to the system designer.A fan running at a particular speed can have an infinite number of operating points all alongits system curve. The fan rpm line will intersect the system curve to produce a single operatingpoint. There can be only one operating point at the intersection of the system curve and the fancurve.Example: System Curve

= 4.0 * (0.0625) = 0.25 in. wg

No two system curves are alike unless constructed

FANS: FEATURES AND ANALYSIS

Using the System Curve

The intersection of the calculated system curve and the fan pressure airflow curve (rpm line)is the rated point (RP). The resulting fan cfm and pressure can then be read.However, in reality the as builtduct system and other factors may result in a system curve with lessresistance or greater resistance (dottedlines) than was estimated.Figure 31 shows a situation wherethe duct system has more resistance toflow than was estimated. The calculated operating point is point CD.However, at the same fan speed andhigher static pressure, the fan will operate at point C>. To get the designairflow at a higher static pressure, it isnecessary to increase the speed of thefan so it will operate at point Q). Assuming the air quantity at C> is 90percent of design, it will be necessaryto increase fan speed by 10 percent.This will result in a large increase infan horsepower, based on the third fanlaw. If the fan motor is already operating near its nominal horsepower rating,it will be necessary to replace it with alarger motor.

Est imated Sy stem Curve

Q)....

Fan Pressure~ Airflow Curve

:::l(J)

~....

0..

cfm

Figure 31Intersection of System Curve and Fan rpm

""'- Estimated System

Curve

Quite often, system designers add

a safety factor to compensate for aninaccurate or incomplete estimate of....system static pressure losses. If the:Iresulting system resistance is less than~....estimated, the fan will operate at point 0... There is no advantage to operatingat this point - the fan may operate at alower efficiency and may require morehorsepower than at design flow. In thiscase, the fan speed should be reduced Figure 32so that the fan will operate at point ~.

, '

Q)

(J)

,,,,

'\.. Less res1stance

means more cfmCoMiant pm lin

cfm

Variation from Estimated System Curve

Commercial HVAC Equipment

FANS: FEATURES AND ANALYSIS

Fan StabilityTo learn about fan stability, we should discuss those factors that lead to fan instability. Faninstability occurs when the airflow through the fan surges or pulses due to turbulent airflow conditions. There are two causes of turbulent airflow through the fan. Incorrect duct connection at thedischarge of the fan is the first cause. Turbulence in the area of the fan ' s cut-off plate can result.Selecting the fan outside of its natural stabilityregion is the second cause. If a fan is operated too farto the left of the maximum static efficiency line, uneven flow through the fan blades can result. The fanmay not be able to maintain stable laminar flow underthese low cfm and high static pressure conditions;turbulence will exist in part of the blade passage.

Flow instability

To avoid instability, care must be taken to select fans operating in a constant volume air system near their maximum efficiency point. When operating in a variable air volume (VA V)system, fans should ideally beselected to the right of themaximum efficiency curve forthe design operating point. Because fans in VAV systemsspend most of their operatinghours at part load, this approachoptimizes the efficiency andhelps ensure that the fan operation does not drift too far to theleft into the naturally unstableAirflow (1 000 cfm)regiOn.Legend' \ - rpm ' \ bhp MSE - Max. Static Eff. SC -System Curve RP - Rated Point

Figure 33Fan Stability - Good Selection

Improper selection and installation can result in noise and

vibration from the fan in the airhandler. Here is an example of apotentially unstable fan selection because it was selected tothe left of the maximum staticefficiency line, well outside ofthe recommended area of operation. For a complete discussionon part load stability, refer toTDP-613 , Fans in VAV Systems.

MSE - Max. Static Eff. SC -System Curve RP- Rated Point

FANS: FEATURES AND ANALYSIS

System Effect, with Example

If there is a difference between what the fan ratings say on paper and what is actually happening in the field , the explanation may be attributable to system effect. System effect refers to theconditions in a duct system that affect fan performance and related testing, adjusting, and balancing work. The subject of system effect usually arises after no allowance or consideration has beenmade for the effect of the duct system connection on the manufacturer' s fan performance.Estimating the impact of system effect is needed in order to arrive at a satisfactory fan selection.

Fan Test Station

Most manufacturers in the USA and Canada rate fan performance from tests made in accordance with the latest AMCA Standard Test Code for Air Moving Devices.AMCA defines exact test procedures and conditions of fan testing sothat fan ratings provided by variousmanufacturers are all on the same basis and can be compared.In general, a fan is installed in aninlet duct test setup as shown in Figure 34. Centrifugal fans are testedwith a discharge duct that is specifiedby AMCA. The duct connection to thefan is idealized to insure accuracy,consistency, and maximum fan performance. Any fan installation thatdeviates from this "idealized" inletduct connection will not be able todeliver rated performance. The impactof system effect reduces fan performance.

Commercial HVAC Equipment

FANS: FEATURES AND ANALYSIS

Fan Velocity Profile

The velocity profile at the outletof a fan is not uniform. The air tendsto hug the outer portion of the fanhousing - providing higher velocitiesat the top of the blast area and lowervelocities at the bottom of the blastarea. A straight section of duct at thefan discharge is required in order toestablish a more uniform air velocityprofile. To calculate 100% effectiveduct length, assume a minimum of 2.5duct diameters. In the diagram thisuniform velocity profile is establishedat the 100% "effective duct length"and mirrors that of the AMCA standard test setup. Failure to provide thisstraight section of discharge duct asshown will result in air turbulence andloss of fan performance.

,,....-ouTLET AREA (TO FINO EQUIVALENT DIAMETER)

To calculate 100% effective duct length, assume a minimum of 2Y. duct

= 5 equivalent duct diameters.

If duct is rectangular with side dimensions

a and b, the equivalent duct diameter is equal to

f4ab

v~

Figure 36Fan Discharge Velocity Profile

Transition to Outlet Ducts

The first section of ductwork is required to transition to the main duct size. This transitionmust follow AMCA rules in order to minimize fan losses. The outlet duct area is to be no greaterthan 107.5 percent or less than 87.5 percent of the fan discharge outlet area. Further, the transitionslope is not be to more than 15 degrees for a converging duct or more than 7 degrees for diverging duct.For 100 percent effective diffusion to a uniform duct velocity the initial transition plus portion of the main duct must extend in a straight line for at least two and one-half equivalent ductdiameters for duct velocities of 2500 fpm or less based on the duct width and height.An additional duct diameter must be added for each additional 1000 fpm. An equivalent ductdiameter for a rectangular duct is equal to the square root of the quantity: [(4 * width * height)/pi].

Losses-Outlet Ducts

Find the Blast Area + Outlet Area Ratio

Because it is virtually impossible

to design a duct system identical tothat used to test the fan, a system effect factor must be determined andadded to the expected system resistance losses.

Outlet AreaHeight

Example: Determination of System

Effect for Outlet Duct on SWSI Fan.

Step 1: Find the fan's blast area to

outlet area ratio.

Figure 37Step ]- Determine Fan Outlet Arrangement

Turn to the Experts." _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _

C_o_m_m_e_r_c_ia_I_H_V_A_C_E_q_u_ip_m_e_nt

22

FANS: FEATURES AND ANALYSIS

Using the ratio of the fan's blast area/outlet area, find the system effect factor in the table (Figure 37). If this is not readily available, assume a ratio of 0.6.

Step 2: Use the Outlet Duct Table to find the system effect factor.To determine the systemeffect factor, use the tableshown.

Air Density = 0.075 lb per cu ft

Commercial HVAC Equipment

FANS: FEATURES AND ANALYS IS

Discharge ElbowsPublished or calculated values for elbowpressure losses according to SMACNA orASHRAE are based on a uniform air velocityprofile entering the elbow. Locate the elbowat least as far away from the fan discharge asthe 100% equivalent length. If this is not possible, elbow losses will be greater thanexpected.If the elbow is located closer than the100% effective length, do not use turningvanes in the elbow. The turning vanes tend tocontinue the non-uniform velocity profilebeyond the elbow.Let's continue our example problem withan elbow "Position B" located at 25%equivalent length. What is the pressure lossof the elbow?

Figure 40Discharge Elbows

System Effect-Discharge Elbow

The table provides theappropriate system effectcurve to be used to determine the added pressureloss imposed by the location and bend of an elbowat the fan discharge.For the example problem, we will use a "B"type elbow located at 25%effective length with aratio of 0.6. This results inthe use of curve "R" toestablish added elbowlosses.

Commercial HVAC Equipment

FANS: FEATURES AND ANALYSIS

Knowing the curve ("R") from the

previous page and the duct velocity of2500 fpm, the added impact of the"Position B" elbow can be determinedto be 0.42 in. wg.

added pressureloss

Air Velocity (fpm

* 100)

Air Density = 0.075 lb per cu ft

Figure 42Elbo w Loss

System Effect Conclusion

As we can see, the easiest way to reduce or eliminate system effect is to avoid discharge ductsituations that would create non-uniform airflows, such as elbows too close to the discharge.The same principle applies to inletairflow arrangements. There are system effect charts that are available tobe used to determine inlet system effect just like we did for the discharge.They are available from sources likeAM CA.Here is an example of nonuniform inlet flow created by ductingthe return too close to the suction of aplenum fan.

System effect caused by non-uniform airflow

into the vortex of the plenum fan

Figure 43System Effect Plenum Fan Inlet

Note

The discussion of system effect in this TDP module

has been limited to the effects of some of the morecommon arrangements influencing fan performance. Fora more comprehensive treatment, reference should bemade to SMACNA or AMCA.

FANS: FEATURES AND ANALYSIS

Miscellaneous Fan Topics

BearingsFan manufacturers use several different types of bearings in their product line. That is because the bearings on a small gas furnace are subject to a different loading than those on a largecentral station air-handling unit. The important terms that one should understand apply to manybearing types. Fan manufacturers work with bearing suppliers to establish a level of quality andassure the bearing life expectance required by the HVAC industry.

Bearing LifeThe life of a bearing is a function of the number of revolutions it experiences before developing evidence of fatigue in the moving elements. The terms that have been used in the industry areB 10 , L 10 and B50 or L 50 . The terms B 10 and L 10 mean the same thing, as do B50 and L50 . The currentterms to be used are L 10 and L50 The American Bearing Manufacturer's Association (ABMA) defines L 10 as the bearing lifeassociated with a 90 percent reliability rate when operating under normal conditions. Normal operation means the bearing was kept clean, properly lubricated, operated at a reasonabletemperature, and free of dust and debris with perfect alignment. In reality, this may not be thecase, so the actual life of the bearing can be shortened based on the application conditions. However, following the manufacturer's installation and maintenance requirements will help extend thelife to the manufacturer's specified values.The designation L 50 indicates the duration in hours that one half (50 percent) of the bearingcan be expected to survive without showing evidence of failure. Conversely, it is the life at whichone half of the bearings can be expected to fail. Thus a bearing with a longer L50 life rating for agiven application can be expected to perform more reliably than another bearing with a shorterL 50 life rating. L 50 life equals five times the L 10 life.To get a L50 life equal to a L 10 100,000 life, you must specify the L50 life to be 500,000 hours.Bearing life is useful whenspecifying a level of bearingconstruction. When required toprovide a given life such as L 10all equipment manufacturersmust supply the same capability bearing for the same givenapplication. A 100,000 hourL 10 bearing will have a lifeover twice as long as 40,000hour L 10 bearing and henceshould last longer on a similarfield application.

FANS: FEATURES AND ANALYSIS

Bearing SelectionMost manufacturers select their bearings as an integral part of the air-handling unit fan design. Some of the main selection criteria include shaft diameter and weight, motor horsepowerrange, weight and location on the shaft, maximum fan speed, fan wheel weight, and the directionof belt pull.Ball bearings with stamped steel housings are well suited for applications with light loads, asin smaller equipment. The use of these bearings is limited to fan products with % inch and smallerdiameter shafts, and one horsepower and smaller motors, such as small fan units.Air-handling units will tend touse ball, spherical, or tapered rollerpillow block or flange-mount bearings. Once the application exceedsthe speed limit for the contact sealand lubrication capabilities of thesolid housing, a pillow block bearingis typically specified. The pillowblock design incorporates a frictionfree seal and a larger grease cavity.Higher speeds can then be attainedand the rollers become the limitingfactor instead of the seal.

Bearing life is the length of time (or number of revolutions)

To enhance accessibility, it is of- Bearing life is affected by several variables.

ten desirable to extend the bearinglubrication lines to the drive side of the fan. In some cases customers want the lubrication linesand fittings extended to the cabinet exterior so that bearing lubrication can be performed withoutstopping the unit. But, customers should also consider the downside of extended lube lines. Bearings should be inspected at the time of lubrication to look for improper operating conditions orsigns of failure. If lube lines fail or vibrate loose, lubricating grease may never reach the bearing,creating an ideal condition for premature bearing failure. Also, bearings can be over-lubricated, inwhich case seals are dislodged, allowing the surplus lubricant to escape.

MotorsHV AC Fan motors typically havetwo types of enclosures: open dripproof (ODP), and totally enclosed fancooled (TEFC) . These two names refer to the method used to cool themotor windings and describe the typeof motor enclosure and internal construction.Electricity flowing through motorwindings develops heat due to theresistance of the windings. This heatis developed continuously and there

Totally EnclosedFan-Cooled(TEFC) Motor

Open Drip Proof

(ODP) Motor

Figure 46Common HVA C Motor Types

Commercial HVAC Equipment

FANS: FEATURES AND ANALYS IS

fore must be removed continuously or the temperature of the windings would rise until the winding insulation bums/ out. ODP and TEFC motors use different methods to remove heat from thewindings.ODP motors have an internally mounted fan pulling ambient air from intake vents in one endof the motor, through the windings, then out of the other end of the motor enclosure through exhaust vents. These vents are placed to prevent falling rain from directly entering the motorenclosure.ODP motors are advantageous because of their low price, availability, and resistance to runaway heating. However, in ODP motors, air is moved directly through the windings, which leavesdeposits on the windings from airborne contaminants such as dust, aerosols, and moisture. Also,splash and wind-driven rain, and even insects and vermin, can enter the motor.TEFC motors have an externally mounted fan covered by a shroud blowing ambient airacross the surface of the motor enclosure. Heat developed in the windings moves by conductionoutward through the motor case then into the air moving along the surface of the motor case. Themotor case is a heat sink drawing heat from the motor interior to the outside. TEFC motors mayhave fins on the motor case enhancing this heat transfer into the air.TEFC motors are advantageous because air is not drawn into the motor for cooling and therefore the windings stay clean and dry. The windings are protected against direct entry of winddriven rain, directed spray, and splash from the ground. Also, insects and vermin cannot enter themotor. TEFC motors protect the single-phase switch keeping it clean and dry.Some single-phase motors have a switch mechanism located next to the windings, which operates the start capacitors and windings. This switch is easily affected by dust, sand, dirt, andcorrosion, and is the largest cause of problems on single-phase motors. ODP motors constantlypull contaminated air over this switch. TEFC motors keep the single-phase switch clean and dry,and therefore single-phase TEFC motors have fewer problems than single-phase ODP motors.Convection through a motor enclosure (TEFC) is less efficient than directly cooling the windings with air (ODP). This makes TEFC motors more expensive to build.Some of the construction differences that make TEFC motors more expensive are: The fan shroud and a higher-grade winding insulation are used to withstand higher temperatures. TEFC motor enclosures are often physically larger than ODP motors Finned motor enclosures cost moreIt is important to note that TEFC motors should never be thought of as "sealed" or "washdown" duty motors, which they are not. TEFC motors are resistant to directed spray, but TEFCmotors are definitely not intended to withstand directed sprays or washing. Air that is heavilyladen with caustic or oxidizing vapors can enter a TEFC motor, but more slowly than an ODPmotor.

FANS: FEATURES AND ANALYSIS

DrivesMost fan drive systems are based on the standard"V" drive belt, which is most commonly used and isrelatively efficient. The use of a belt drive allows fanrpm to be easily selected through a combination of motor rpm and drive pulley ratios. Multiple belts andmultiple-groove sheaves are required to meet higherhorsepower requirements. Drive ratio is defined as follows:

=Motor Output/Motor Efficiency

Required Motor Output= (Fan bhp) +(Drive Losses)

Drive Losses increase required motor output by 3 to 5%

Figure 48Motor and Drive Terminology

The fan drives are either fixed

drive or adjustable drive. When a unitis furnished with an adjustable drive,the fan sheave diameter can bechanged to fine tune the fan speedand performance.Drive losses refer to the inefficiencies resulting from the frictionaleffects of pulley and belt assembliesbetween the motor and the fan wheel.Higher belt speeds tend to havehigher losses than lower belt speedsat the same horsepower.Drive losses are based on theconventional V -belt, which has beenthe most commonly used drive in theindustry for several decades.

For example, fan brake horsepower output is determined to be 17.1 bhp. What is the requiredmotor output horsepower?The belts are V -types, the drive loss is 5%.Drive loss

= 0.05 X 17.1 hp = 0.86 hp

Motor power output

17.1 bhp + 0.86 bhp = 18 bhp

Commercial HVAC Equipment

FANS: FEATURES AND ANALYSIS

Spring IsolationThe presence of vibration is not desirable in any piece of mechanical equipment, and fans areno exception. Excessive fan vibration can cause premature failure of critical parts that may resultin high maintenance cost anddowntime. Consequently, itis common to find a "vibration clause" written intomany specifications.The causes of vibrationmay be vibration that is aresult of an unbalanced fanwheel or vibration causedfrom drive misalignment,belt tension, bent fan shaft,etc.

Standard 2-inchSteel Spring Isolator

2-inchSeismic Rated Isolator

To alleviate problems Figure 49

caused by vibration, manu- Fan Spring Isolationfacturers may supply internalspring isolation as part of thefan assembly. Most fan assemblies are dynamically balanced before they are installed in the fancabinet. This ensures that the assembly does not suffer from rotating part unbalance.

SummaryThe objective of this module has been to familiarize the designer with fans and how they represent a very important segment of the typical air conditioning system. A clear understanding ofhow they operate is an essential part of being able to design a good system.System effect exists on most projects as a result of the fan being installed somewhat differently than laboratory test conditions. Since we cannot always prevent these differing conditions,we must account for their system effect.An examination of the types of fans available and their performance prepares a system designer to evaluate fan system performance. While it is important that system designers understandthe intricacies of fans and fan selection, there is software available to aid in the determination ofwhat fan should be used in a specific application. It is also important to understand motors,drives, and bearings, which are an important component of any fan selection and HV AC system.

Commercial HVAC Equipment

FANS: FEATURES AND ANALYSIS

AppendixFan Law EquationsFan Laws for Constant Mass Flow - Capacity, speed and pressure vary inversely as the airdensity, that is, inversely as the barometric pressure and directly as the absolute temperature.a.

b.

c.

cfm z

llll

* [ DENSITYsm

cfm 1

DENSITYAcT

rpm 2

P52

d.

= rpm * [ DENSITYsm1

DENSITYACT

= P5 * [ DENSITY5 m1

DENSITYACT

bh 2 = bh * [ DENSITYsm'Pp,DENSITYACT

or cfm 1 *

[TTACTm l5

or rpm 1 *

[TTACTm l5

ll

TACT* [-

or P5

T5 m

or bh 1 * [TACT'PTSTD

Fan Laws for Constant Volume (cfm) and Fan Speed - Horsepower and pressure vary directly with the air density, that is directly as the barometric pressure, and inversely as the absolutetemperature.a.

b.

bhp 2 =bhp , *[DENSITYAcr] or bhp

1DENSITY5mP.s 2 -_ P.s ,.. [ DENSITYACT1DENSITY5m

* [TsmTACT

or P.s ,.. [ T5 m1TACT

Fan Laws for Constant Static Pressure - Speed, volume flow (cfm) and horsepower vary inversely as the square root of the density, that is, inversely as the square root of the barometricpressure and directly as the square root of the absolute temperature.

FANS: FEATURES AND ANALYS IS

Work Session Answers

1.

A forward-curved fan because the airflow through the fan increases at a constant rpm.

2.

centrifugal, and axial

3.

A forward-curved fan wheel is fabricated of lightweight and low cost materials, and has 2464 shallow blades with both the heel and the tip of the blade curved forward. The airfoilblades are more ruggedly built, adding to their weight, and are curved backward with 8-18blades.

4. 2.5 equivalent duct diameters.

5. System effect refers to the conditions in an actual duct system that affect fan performance andrelated testing, adjusting, and balancing work. It can be prevented to a degree, but must beaccounted for.6. L 10 rated bearings have a 90% reliability of their stated amount of time, generally expressed inhours. That is, 90% of the bearings with L 10 ratings will not have developed metal fatigue after their designated life span. L50 rated bearings have an 50% reliability of their stated amountof time; only 50% of a group of identical bearings with an L50 rating will not yet have developed metal fatigue.7.

cfm varies directly with fan rpm

static pressure varies with the square of the fan rpmbrake horsepower varies with the cube of the fan rpm

8.

A plenum fan builds up static pressure in the plenum. It does not propel the air down a singleduct opening. The contractor cuts discharge openings in the plenum and the air exits underthe static pressure developed by the fan.

9. The filters load up (collect dirt) increasing pressure drop, which changes the resistance of thetotal system. Someone may change the position of a balancing damper, which changes thesystem curve.10. A vane axial fan incorporates a straightening vane assembly. This helps to make it more efficient than the tube axial fan.

Learning Objectives:After reading this module, participants will be able to:

Identify fans types that are used in the HVAC industry, their operating characteristics, andbasic construction.Understand the application limitations for types of fan impellers.Utilize the fan laws to construct a system curve for a typical system.Identify stable fan selections using fan curves.Calculate the system effect for an example fan operating condition.Understand fan bearing life, fan drives, and fan isolation techniques.

Each TOP topic is supported with a number of different items to meet the specific needs of theuser. Instructor materials consist of a CD-ROM disk that includes a PowerPointTM presentationwith convenient links to all required support materials required for the topic. This always includes:slides, presenter notes, text file including work sessions and work session solutions, quiz andquiz answers. Depending upon the topic, the instructor CD may also include sound, video,spreadsheets, forms, or other material required to present a complete class. Self-study or studentmaterial consists of a text including work sessions and work session answers, and may alsoinclude forms, worksheets, calculators, etc.